Hello LKNDY. I've moved your question to the HTC Hero troubleshooting area for you.
You may need to calibrate your battery to your device by allowing it to drain completely then fully charging it a few times. There are other, more elaborate methods of doing this, but merely draining it to a black screen and then charging should take care of how the device sees the battery levels, giving you a more accurate battery meter. This happens naturally over time with usage and charging cycles, as long as you're not always "topping" the battery (which is ok as long as it's calibrated to the phone for accuracy, topping does not harm a Lithium-Ion battery as it does a Nicad).
Eight percent battery loss seems a bit more than normal, but not really excessive, depending upon usage during that 20 minutes. If no usage other than what you have configured to update or sync in the background, then you may have an excessive battery drain issue.
First check to see if apps are updating by looking at things such as weather and news apps menu configurations, etc.
Also, very fast battery drainage can be caused by a "sync loop," if you've got gmail, facebook and other syncing configured to auto sync.
Auto sync should not cause quick drainage, but sometimes that loop will occur and the data and caches for all synced apps need to be cleared in menu > settings > applications > manage applications.
After doing that, reboot your device and reconfigure each app because clearing data removes your personal settings.
If this does not remedy the problem, you may have a defective battery.. and that means replacing it.
Good luck. Please post back about how things are going.